An optical communication system using a submarine cable (hereinafter referred to as a “submarine cable system”) requires flexible network construction capable of reducing an influence on communication when a trouble occurs in the submarine cable, changing a future network configuration, and the like. As one example of the solution, there is a method of disposing an optical filter and an optical switch in a submarine branching device, combining the optical filter and the optical switch, and changing an add/drop ratio of an optical signal by remote control.
FIG. 10 is a block diagram illustrating a configuration of a general submarine cable system 90. The submarine cable system 90 includes a submarine branching device 900, terminal stations 901 to 903, and submarine repeaters 904 to 906. The terminal stations 901 to 903 are devices that are installed on land and terminate submarine cables. The submarine branching device 900 and the submarine repeaters 904 to 906 are installed on the seabed. For transmitting data, in the submarine cable system 90, a wavelength division multiplexing (WDM) signal is transmitted by an optical fiber built in the submarine cable. The submarine branching device 900 and the terminal stations 901 to 903 perform interactive transmission of the WDM signal therebetween. The submarine repeaters 904 to 906 each amplify an optical signal propagating between the terminal stations 901 to 903 and the submarine branching device 900 by a built-in optical amplifier.
The submarine branching device 900 includes optical filter modules 911 to 914 and optical couplers 931 and 932. The optical filter modules 911 to 914 each include an optical switch and a plurality of optical filters inside. The optical switch switches an optical path to the plurality of optical filters.
The WDM signal sent from the terminal station 902 is amplified by the submarine repeater 905 and received by the submarine branching device 900. The received WDM signal is branched into two signals by the optical coupler 931 and the signals are input to the optical filter modules 911 and 912.
The optical filter modules 911 and 912 each switch the optical filter to be used by the optical switch. Therefore, a combination of wavelength bands of a WDM signal transmitted from the terminal station 902 to the terminal station 901 and the terminal station 903 can be changed by controlling the optical filter modules 911 and 912. For example, the WDM signal sent from the terminal station 902 can be controlled in such a way that all the WDM signals are sent to the terminal station 901 by controlling the optical filter modules 911 and 912. Alternatively, the WDM signal sent from the terminal station 902 can be controlled in such a way that an optical signal as a part of wavelengths of the WDM signal is sent to the terminal station 901 and an optical signal as the rest of wavelengths is sent to the terminal station 903.
On the other hand, the WDM signals sent from the terminal station 901 and the terminal station 903 are respectively amplified by the submarine repeaters 904 and 906 and received by the submarine branching device 900. The received WDM signals are subjected to processing by the optical filter modules 913 and 914, coupled by the optical coupler 931, and sent to the terminal station 902.
The optical filter modules 913 and 914 can switch a combination of wavelength bands of the WDM signals transmitted from the terminal station 901 and the terminal station 903 to the terminal station 902. For example, control can be performed in such a way that only the WDM signal sent from the terminal station 901 is sent to the terminal station 902 by controlling the optical filter modules 913 and 914. Alternatively, control can be performed in such a way that an optical signal as a part of wavelengths of the WDM signal sent from the terminal station 901 and an optical signal as a part of wavelengths of the WDM signal sent from the terminal station 903 are coupled and sent to the terminal station 902.
In other words, an add/drop ratio of the submarine branching device 900 can be changed by controlling the optical filter modules 911 to 914.
In relation to the present invention, PTL 1 describes a technique for suppressing occurrence of an optical surge during a switch operation by setting a time constant to be greater in an operation of an optical switch.